Today the forced cooling of a static electric induction system such as a power transformer or reactor is usually performed at a steady state with a constant cooling fluid flow rate.
There are three main modes of heat transfer involved in the cooling of the induction system, e.g. of the conductor windings thereof. Conduction in the conductor, diffusion from the surface of the conductor to the bulk of the cooling fluid and convection by the fluid stream. During the conduction phase there is a time lag to transfer the heat from, e.g., the middle of the conductor to its surface. The diffusion is very slow for laminar flows but gets substantially faster when the flow structure becomes turbulent or contains inherent instabilities. The convection time scale corresponds to the ability of the fluid and flow to carry the heat from a point situated in the bulk to a point downstream. In general, the conduction time constant is by far larger than the time constants needed by convection and turbulence or instabilities induced diffusion.
It is known to temporarily increase the flow rate of the cooling fluid in response to a temperature increase in the fluid. For instance, JP 2006/032651 discloses the use of an insulating medium circulation flow rate increasing means which is able to temporarily increase the flow rate of the insulating/cooling medium above a steady-state flow rate upon detection of a temperature increase in the insulating medium in an electrical apparatus with an iron core and winding.
However, to merely measure a temperature of the insulating medium is not sufficient to determine the occurrence of any hotspots within such an electrical apparatus. The outlet temperature of the insulating medium only gives a general measure of the amount of heat exchanged, not a measurement of how efficient or uniform the heat exchange is.